added preserve-extensions functionality

[tjb1982]

This helps in situations where you have two files with the same name, differing only by extension, which happens a lot in C/C++/Objective-C, etc.

e.g. you might have some files like this:

• bar.c
• bar.h
• foo.c
• foo.h
• main.c

which groc generates as:

• bar.html
• bar.html
• foo.html
• foo.html
• main.html

In order to prevent ambiguous filenames, preserve the extension:

• bar.c.html
• bar.h.html
• foo.c.html
• foo.h.html
• main.c.html

[RussellChamp]

I'd really like to see this get added as I'm trying to groc some .cpp and .h files that have the same names